This invention enables the preparation of glazes without lead oxide additions but with the properties that make lead oxide glazes so desirable and widely used.
Glazes are thin silicate mixtures fused on the surfaces of ceramic bodies such as clay wares. They are glass-like in physical and chemical nature, thus they are hard, suitably insoluble (except in strong acids and bases) and impermeable to liquids. Like glasses, glazes are not definite chemical compounds but complex mixtures or solutions. Glaze compositions are known as either raw glazes or fritted glazes. Raw glazes are batched, possibly milled, and directly applied to the surface of the ceramic ware prior to firing (heating to a high temperature) to melt and fuse the glaze upon the ware which may or may not have been previously fired itself. Fritted glazes are glazes that are melted or fused, quenched, and milled prior to application to the ware. The advantages of the extra melting or fusing step are several including, for example, the ability to use water soluble ingredients, ingredients that may be harmful to contact if raw and ingredients that require additional lengths of time to fuse.
Lead oxide is an especially advantageous ingredient of glazes. It provides for excellent gloss, smooth surfaces and transparency. Lead oxide is a low temperature flux reacting readily with silica, alumina, and other glaze ingredients. It forms melts of low viscosity. Enclosed bubbles are more easily released by lead oxide glazes. Blisters and defects are more easily healed and obscured. Lead oxide glazes tend to have a favorable viscosity lengthening the firing range thus reducing the risk of overfiring. The higher contact of alkali necessary to produce the desired fusibility in the absence of lead oxide tends to raise the thermal coefficient of expansion thereby promoting crazing. Any attempt to produce similar desirable properties without the use of lead has proved to be a difficult technical problem. See Ceramic Glazes by Cullen W. Parmelee (Second Edition, 1951).
Lead glazes are not without drawbacks; for example, improper handling of lead containing raw materials can comprise a health hazard. Further, improperly compounded lead containing glazes when in long contact with food (fruit and vegetable juices) can result in the food becoming unsafe for use.
While molybdenum (as an oxide) has been considered for use in special ceramic glazes in order to provide flowability and wettability, it has not been widely used. In U.S. Pat. No. 3,927,238, it is taught that amounts of MoO.sub.3 above 0.5 percent reduces its effectiveness.
The applicants have discovered that the small quantities of alkaine earth molybdenates may be suitably used in lead-free glazes either as a frit addition or as a mill addition to produce glazes having the properties of lead oxide containing glazes.